In magnetic data storage systems it is necessary to detect certain electrical fault conditions during data writing operations that will destroy data or cause it to be irretrievably lost. In magnetic disk data storage devices, data recording is accomplished by switching the direction of a current across the write head of the device. Three fault conditions which may affect such systems are: (1) lack of write data transitions, (2) shorting of the write head wire to the support, and (3) lack of write current. The first condition typically arises from hardware problems; conventionally used data encoding schemes require that a transition be provided within a preset period of time or the data is invalid. The second condition typically arises from a break in the insulation of the head wire. The third condition also typically arises from a hardware fault; if no write current is generated, then no data will be written even though all other indications are that valid data was written. Any of the three aforesaid fault conditions will prevent the proper recording of data and may result in its eventual loss.
Older data disk systems typically utilize ferrite write heads, while newer drives often incorporate thin film write heads. Thin film write heads differ from the older, ferrite write heads in that the former are mostly resistive elements with very little inductive component while the latter are almost entirely inductive. One sees alternating current spikes of several volts across the ferrite head when writing whereas one sees only millivolt level potential switching across the thin film head when writing.
Various methods have been employed in the past for detecting one or more of these fault conditions in various types of magnetic data storage write systems. For example, U.S. Pat. Nos. 3,080,560, 2,789,026, 2,919,968 and 3,315,268 all use a write head current or signal induced from a head current to check proper write operations. In particular, each uses a timing arrangement that checks the write current at each possible data write point (current transition point) for the occurrence of a transition in the write current (or the lack thereof). After each check, the circuit is reset to monitor for the next potential transition at the next potential transition point. Because many encoding schemes do not require a transition at each potential transition point, additional logic is typically provided to determine if more than an allowable number of potential transitions have been missed. While these disclosed circuits may detect one or even two of the aforesaid three faults, none is responsive to all three of the aforementioned faults. Also, these testing schemes and devices are not easily adapted to thin film head systems.
U.S. Pat. Nos. 3,559,192; 3,617,652; 3,668,310 and 4,212,702 utilize another well known technique of write fault detection: and immediate subsequent read operation to verify that the write operation has generated a proper magnetic record on the data storage media. Reading may reveal all three faults. However, the fault may be such that the quality of the written data is poor, but readable. This condition will typically worsen with time and it is preferable to check the data quality while it is being written.